Go for it - you'll be able to find some great looking pieces, and you can't beat the pricing.
You might want to try and select pieces that look more weathered, don't have bark, etc.
Since it's freshwater, you don't have to worry about trying to remove the salts, but it's probably still a good idea to soak them to help remove excess tannins/etc., and to help them sink.
Some people advocate doing soaks with bleach, etc. I'm not sure this is really necessary, but it probably depends on your source, and what critters you have/plan on having.